In the lighting field, the phase-controlled (chopping) dimming method using Triode for Alternating Current (TRIAC) is a dimming method most widely nowadays, and is widely used in the fields such as stage lighting and surrounding illumination, since it has the advantages of the small size, the reasonable price and the wide dimming power range. By applying the TRIAC phase-controlled operating principle, the supply voltage of the lamp circuit is controlled by controlling the conduction angle of the TRIAC element to chop out a part of the sine wave voltage input from the AC power network, to decrease the average value of the output voltage, so as to achieve the dimming function for LEDs.
FIG. 1A illustrates a principle block diagram of a dimming circuit adopting the existing TRIAC phase-controlled dimming method, which comprises an AC input power supply 101, a TRIAC 106, a trigger circuit 113, a rectifier bridge 107, a holding resistor 108, and a filter circuit consisting of a diode 109 and a filter capacitor 110. Wherein, the trigger circuit 113 may consist of a few parts, such as a DIAC 105, an adjustable resistor 102, a resistor 103 and a capacitor 104. The anode of the TRIAC 106 is coupled to one end of the AC input power supply 101, and the cathode thereof is coupled to the rectifier bridge 107; the point A in the trigger circuit consisting of the adjustable resistor 102, the resistor 103 and the capacitor 104 connected in series in turn is coupled to the control electrode of the TRIAC 106.
During the operation, by adjusting the resistance value of the adjustable resistor 102, the voltage of the control electrode of the TRIAC 106 is changed, and the TRIAC element 106 is turned on when the voltage of the control electrode is greater than a certain voltage, so as to control the phase of the conduction angle of the TRIAC element 106 with respect to the AC input power supply 101. It can be seen, from the schematic diagram of the operating waveform of the TRIAC phase-controlled dimming with leading edge triggered, as shown in FIG. 1B, that at a certain time point t1 (or a certain phase wt1) after the zero crossing point of the voltage of the sine AC input power supply 101, a positive triggering pulse Vtrg1 is applied to the control electrode of the TRIAC element 106, so that the TRIAC element 106 is turned on, which will maintain until the positive half-sinusoidal wave period ends according to the switching characteristics of the TRIAC element. Therefore, in the positive half-sinusoidal wave period (i.e., the section of 0˜π), the TRIAC element 106 is turned off within the range of 0˜wt1, i.e., the control angle α, while the TRIAC is turned on in the phase section of i.e. wt1˜π, the conduction angle φ. In the same way, in the negative half-sinusoidal wave period of the AC power, a triggering pulse Vtrg2 is applied at the time point t2 (i.e., the phase angle wt2) so that it is turned on. It goes round and round, and it is controlled to be turned on in each half-sinusoidal wave period to get the same conduction angle. The magnitude of the conduction angle φ (or the control angle α) of the TRIAC can be changed by changing the triggering time (or phase) of the triggering pulse, so as to generate an AC voltage Vacin with phase loss, which is rectified by the rectifier bridge 107 to generate the DC voltage Vdcin, and it can be seen that the greater the conduction angle φ, the higher the output voltage Vac of the circuit, and the light from the load 112 becomes brighter by controlling the load 112 through the SMPS (Switching Mode Power Supply) driver 111.
However, the filter capacitor 110 needs to be arranged in the dimming circuit for LEDs adopting the prior art as shown in FIG. 1A. Because the capacitance of the filter capacitor 110 is high, the LC resonance in the circuit causes that the current through the TRIAC is uncontrollable, and the TRIAC cannot maintain the turned-on state, and therefore flicker occurs in the LED. In addition, the electronic transformer in the traditional circuit is originally designed for a halogen lamp rather than LED lamp, and it requires a minimum holding current to operate normally during the operation thereof. Therefore, in the LED driving circuit with the electronic transformer, the problems of the uncontrollable current and the turning off of the electronic transfer are resolved by a method for adding a dummy load (for example, the holding resistor 108); however, the power consumption of the added dummy load causes a lower operating efficiency.
Therefore, in the LED driving circuit with electronic transformer, it is a challenge how to both satisfy the requirement of the minimum current of the electronic transformer, to dim the LED load accurately so that no flicker occurs in LED load, and to keep the operating efficiency of the whole system high.